Description

Since I've moved into this house, I've noticed that the first shower of the day is colder than the any subsequent shower. More specifically, if no hot water has been used for a while, the water becomes more luke warm than hot. However, the maximum temperature of the water after it recovers from this issue is plenty hot.

To work around this problem, my wife and I use my dishwasher's timer feature and run it over night. This usage of hot water in the middle of the night helps the water be a more reasonable temperature in the morning. But we don't always have a load of dishes to do each night...

I've got some theories, and they're mostly related to the thermostat and/or it's position. I keep the temperature between the A and B settings (I'm in the US, if that matters). The water heater is only about 7 years old and I have flushed it recently. There was no difference after flushing it.

My Theories

The thermostat is malfunctioning. The thermostat most likely has a high temperature shutoff (where it turns the heat off once it achieves the desired temperature) and a low temperature turn on (where it turns the heat on once the water gets too cold). One theory is that the slow cooling of the water over time isn't enough to trip the lower threshold on the thermostat. However, the cold water coming in from the main is enough to shock the thermostat and turn it on.

My second theory is that the thermostat is at the bottom of the hot water heater (I'm pretty sure it is) and the pilot light is keeping the water in that area hot enough that it doesn't kick on the heat. Since I keep the temperature at a moderate setting (I have young children), I'm thinking that the water at the bottom of the tank might never get cold enough to kick on the heat because of the proximity to the pilot light. However, when the tank starts to drain and the cold water from the main comes in, it does trip the thermostat.

Any idea if either of these theories might be right? How can I fix it? Do I just need a new water heater? Will that even help?

What type of hot water heater do you have? Is it tankless,indirect tank, electric?
–
mikesDec 31 '12 at 19:58

@mikes Sorry, I forgot to mention that part. It's a gas tank water heater. (Though the talk of the pilot light should have given that away.) It's probably around 40-50 gallon. It's a State Select brand. Pretty much a basic builder grade unit.
–
LukeDec 31 '12 at 20:27

I doubt it's the pilot light, but the thermostat could have an excessively large range.
–
BMitch♦Dec 31 '12 at 21:26

4 Answers
4

This manual for a State Select gas-fired water heater states "Due to the nature of the typical gas water heater, the
water temperature in certain situations may vary up to
30°F higher or lower at the point of use such as, bathtubs,
showers, sink, etc.
This means that when the temperature adjustment dial is
set at the mark approximating 120°F, the actual water
temperature at any hot water tap could be as high as
150°F or as low as 90°F." (page 16).

Given your low temperature setting, it seems pretty likely that you are experiencing the expected behavior for this unit. You might consult a plumber to see if you could get a more consistent temperature by setting the thermostat higher and installing a mixing valve to regulate the temperature to your desired level. This might also give you an option to have hotter water available to certain locations (i.e., dishwasher) that don't present a risk to your kids.

First shower you have a good chance of tapping the heater in the lower half of its temperature range. Second shower happens after heater heats water to top of temperature range.
Hysteresis is built into the circuit so that the heater doesn't cycle on an off rapidly trying to maintain the water in a small temperature range.
Some thermostats include a hysteresis adjustment that lets you set what temp range is allowed.

"Some thermostats include a hysteresis adjustment that lets you set what temp range is allowed." - Do you know of any gas fired water heaters that use a thermostat that provides a hysteresis adjustment? Nearly every water heater manual I've looked at provides an almost identical statement about a +/- 30 degree differential.
–
LukeJan 3 '13 at 19:33

I think you have a grasp of the situation. I think it's common for water heaters to have a large temperature swing. It keeps them from having to cycle on and off frequently. When they are overly hot, it's not an issue. Turned down quite a bit and the "feature" is quite noticeable. A replacement would likely be similar. You might try turning it up a little bit, but not so that it's truly dangerous. Once your kids are old enough to understand hot, you can turn it up even more.